This invention relates to an exposure control device for a photographic camera which uses a known, conventional, exchangeable lens and is provided with a light measuring device adapted to measure scene light which has been transmitted through an objective lens and a diaphragm, and more particularly to such a device which, at a pre-selected shutter speed, effects the exposure control by adjusting the diaphragm commensurate with the pre-selected shutter speed and then controlling the shutter speed commensurate with the adjusted diaphragm.
With the conventional exposure control mechanism set at a shutter speed beforehand, in which the diaphragm aperture is determined according to the position of a meter pointer, an error may occur in the diaphragm aperture setting, if the meter pointer is positioned midway between two adjacent positions due to the pointer holding member which should hold the pointer at its correct deflected position. Moreover, with a conventional exposure control of the type, in which an electromagnet locks the diaphragm which is to be actuated electrically, an error is likely to occur in the adjustment of the diaphragm aperture, because of the timing lag between diaphragm-locking and the operation of the electromagnet. In either case, whether the diaphragm aperture is controlled in accordance with the position of the meter pointer, mechanically or electromagnetically, the shutter is operated at the selected speed, irrespective of the error in the diaphragm aperture adjustment and thus, an error is necessarily introduced into the exposure to be controlled.
Also, with the conventional exposure control device for controlling exposure by adjusting the diaphragm aperture, based on a pre-selected shutter speed, there still remains a problem. Namely, optimum exposure may not be obtained, even if the diaphragm aperture is adjusted to a minimum aperture value at a selected shutter speed. Conversely, when the photographic object is too dark, there may not be obtained an optimum exposure, even if the diaphragm is fully opened at a selected shutter speed. In such instances either excessive exposure or insufficient exposure will result, because the shutter will be operated at the selected speed, even if the diaphragm is adjusted to its minimum aperture value or even if the diaphragm is fully opened.
Accordingly, the present invention overcomes the aforementioned errors which are necessarily inherent in a conventional diaphragm control device used in an exposure control device of the type in which the diaphragm aperture is controlled and the shutter speed is selected beforehand.